Devil’s Postpile (US), Fingal’s Cave (UK), Giant’s Causeway (Ireland) – these are nature’s beauties because of their spectacular columns. But how do columns form? Well this illustrated and short blog post is a selective stravaig through the charming world of columns, with a focus on lava-ice and lava-water interactions. It also provides a bit of insight into the science behind column formation, as well as hinting at unexplored areas and discoveries yet to be made.
The massive interiors of lava flows often show different zones, and where a zone consists of well-developed columns it is given the term ‘colonnade‘, and the simplest of lavas has an upper colonnade and a lower colonnade which meet (usually imperfectly) towards the middle of the flow. ‘Entablature‘ is the name given to a zone displaying irregular columns oriented in various directions, or a zone comprising much smaller blocks (known as ‘cub-jointed’ lava or ‘kubbaberg’). Entablature is interpreted by all workers as representing penetration of additional coolant (water/steam) into a lava flow, and thus is a useful environmental indicator.
One of the plus points of being an academic and having a bit of time to do research is that you can get a good PhD student working on something that really fascinates you. For many years I have looked at columns and other fractures in lavas, especially those involving volcano-ice interactions, and I knew there was some good science to do on them. So I managed to persuade a couple of colleagues that a PhD project entitled ‘Lavas in Glacial Settings’ would be a winner, and of course you cannot study these lavas without being surrounded by columns. So part of the project became an in-depth look at columns and fracture formation in lavas, and we had some nice surprises and made some new discoveries.
I am going to show some examples of fractures that illustrate a number of points. But first, some basics about columns. For those who want further information I have added ‘geek notes’ but you really don’t have to read these to understand and enjoy the images.
- Columns form due to thermal contraction of cooling lava.
- They form at right angles to the cooling surface, so a horizontal lava body (e.g. lava flow or sill) will have vertical columns, whereas a vertical intrusion (e.g. dyke) will have horizontal columns. [Geek note: Columns propagate in the direction of the thermal gradient defined by the isotherms in the cooling lava body. In a horizontal lava body the isotherms will be horizontal, and therefore the columns will propagate vertically.]
- Column diameter is related to cooling rate, so for example a faster cooling rate will produce smaller columns. [Geek note: Column diameter is controlled by the viscoelastic response of lava to cooling, so faster cooling leads to smaller-diameter columns. There is as yet no widely-accepted model which relates column diameter to cooling rate. There’s a PhD project for you…!]
- Columns form in lavas of all compositions, though basalt columns are best studied. [Geek note: Rheological factors mean that basalt columns best approach the ‘equilibrium’ condition which is the formation of hexagonal columns with equal sides. Rhyolite columns get nowhere near this ‘equilibrium’ condition.]
- Columns do not form smoothly – they form in distinct steps. This process is reflected in the ‘steps’ that can be seen on many columns, and on vertical columns these ‘steps’ are horizontal. These have various names, such as ‘chisel marks’, ‘striae’, ‘chatter marks’ and ‘step-wise advance cracks’. I’ll call them ‘chisel marks’ in this blog. [Geek note: These form parallel to the long column axis. They are beautiful examples of cyclic fracturing in a uniform stress environment where fracture propagation is retarded until further cooling and contraction enables the tensile strength of the lava to be overcome. One it has been overcome the fracture is initiated and propagates from cool into hotter lava, but stops as the fracture encounters lava too hot to fracture in a brittle manner. Then the process starts again.]
- Small but subtle features at the chisel marks enable the direction of formation of the columns to be determined. [Geek note: these ‘hackle’ marks and plumose structures identify the point of fracture initiation in the cooler part of the lava and the direction of fracture propagation into the warmer part of the lava.]
- In a horizontal lava body such as a lava flow or sill, cooling occurs both at the top and that the bottom, so columns grow upwards from the bottom and downwards from the top. They meet in the middle, often surprisingly well, though an accommodation zone of imperfect joins is usually present. [Geek note: The upper columns are usually longer than the basal columns, suggesting that cooling from above is more effective than cooling from below. This is not surprising, as convective removal of heat from the top is generally more efficient than conductive removal of heat from the base.]
- Not all lava bodies have columns, and the reason why they don’t is probably another PhD project….